It is inevitable that the start/end of a seam will loosen up as you work with the blocks, especially if it is handled multiple times. I like to leave a good tail end of thread (at least an inch or so) until the complete top is done and then I clip threads close. If a seam begins to loosen while I'm working, I just have to tug on the thread and tighten up again. I clip my threads when I do my last pressing.